1. Technical Field
The present invention relates to a technology that reduces display defects, such as flickering and screen burn in an electro-optical device.
2. Related Art
A liquid crystal display is a device that modulates transmitted light or reflected light to display an image by controlling a voltage applied to a liquid crystal for every pixel and controlling the transmissivity or reflectivity of the liquid crystal. In the liquid crystal display, a voltage applied to the liquid crystal is controlled by the liquid crystal being interposed between a pixel electrode provided with respect to each pixel and an common electrode common to the pixels and by a voltage being controlled between each pixel electrode and the common electrode. In an active-matrix-type liquid crystal device, each pixel electrode is connected to a signal line (referred to as a data line) via a corresponding switching element (usually known as a field effect transistor, but hereinafter referred to as a transistor for short) and a potential according to the potential supplied to the signal line (referred to as a display signal) is written into the pixel electrode when the transistor is in an ON state. The potential of an common electrode is usually controlled in such a manner as to become a mostly constant potential. Moreover, the potential of each component of the liquid crystal display is expressed by a potential difference (a voltage) between that potential and the potential defined as a reference (for example, ground potential). Therefore, in the following description, potential and voltage are used to have the same meaning.
In the liquid crystal display, an alternating current drive that periodically changes the polarity of the voltage applied to the liquid crystal is performed because deterioration in the liquid crystal occurs when a direct current voltage is applied to the liquid crystal for a long time. A state where the potential of the pixel electrode is higher than the potential of the common electrode is referred to as a state where a voltage with a positive polarity is applied to the liquid crystal, and a state where the potential of the pixel electrode is lower than the potential of the common electrode is referred to as a state where a voltage with a negative polarity is applied to the liquid crystal. In the alternating current drive, a signal in which the positive polarity and the negative polarity appear alternately (for example, every frame) is supplied with respect to a predetermined central potential, as a display signal supplied to the data line, and the potential of the common electrode is set in such a manner as to usually match the central potential of the display signal.
It is known that a phenomenon called feed-through occurs in the liquid crystal display described above. Feed-through is a phenomenon where the potential of the pixel electrode is changed from the potential that is written when the transistor is in a ON state, when the transistor changes from in an ON state to in an OFF state, due to a parasitic capacity between a gate electrode of the transistor and an electrode (for example, a drain electrode) connected to the pixel electrode. The direction in which the potential of the pixel electrode is changed by the feed-through is a constant direction (is the downward direction when the transistor is an N-channel type and is the upward direction when the transistor is a P-channel type), regardless of the value of the potential written into the pixel electrode. Because of this, the central potential of the pixel electrode deviates only by the potential change due to feed-through, from the central potential of the display signal supplied onto a signal line. Therefore, the direct current voltage component acts on the liquid crystal, by the potential of the pixel electrode being changed due to feed-through, in a case where the potential of the common electrode is set in such a manner as to match the central potential of the display signal supplied to the signal line. In other words, an unbalance occurs in the voltage with the positive polarity and the voltage with the negative polarity which are applied to the liquid crystal. This becomes a cause of the occurrence of deterioration in the liquid crystal, screen burn, and flickering and the like. JP-A-2002-189460 discloses that the potential of the common electrode is shifted from the central potential of the display signal only by the potential change in the pixel electrode due to feed-through.
JP-A-2009-175563 discloses that two signal lines are arranged in such a manner as to at least partly overlap each other via an insulation film (that is, multi-layer wiring is performed), in the liquid crystal display having the two signal lines with respect to each pixel column. In each pixel column, the pixels in the odd-numbered rows are connected to one of the two signal lines via the transistor, and the pixels in the even-numbered rows are connected to the other of the two signal lines via the transistor. The pixels in the odd-numbered rows and the pixels in the even-numbered rows are different in terms of the area of the pixel electrode and one composite pixel is formed by a pair of pixels adjacent to each other in the column direction.
The optimal potential of the common electrode (a opposing electrode) may differ in the pixels in the even-numbered row and the pixels in the odd-numbered row, in the liquid crystal display disclosed in JP-A-2009-175563. Because of this, for example, when the potential of the common electrode is set in such a manner as to be of an optimal value with respect to the pixels in the odd-numbered row, the potential of the common electrode is not of an optimal value with respect to the pixels in the even-numbered row. As a result, the unbalance may occur in the voltage with the positive polarity and the voltage with the negative polarity that are applied to the liquid crystal and defects such as deterioration in the liquid crystal, screen burn, and flickering may occur.